Khondalite Belt Research Articles

LA-ICP-MS U–Pb zircon ages of the Qianlishan Complex: Constrains on the evolution of the Khondalite Belt in the Western Block of the North China Craton

The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207Pb/ 206Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.

Petrological and geochemical characteristics of Marunthurkota syenites from the Kerala khondalite belt, southern India

This note reports new occurrences of syenite bodies around Marunthurkota area from the Kerala khondalite belt (KKB). Petrological and geochemical studies suggest that the syenites have a pronounced A-type affinity, metaluminous characteristics with high concentrations of alkalies, Rb, Sr, Zr, and high K 2 O/Na 2 O ratio. Miaskitic nature (agpaitic index<1) of syenite suggest involvement of CO 2 related phase in their genesis. The petrological characteristics signify crystallization of the rock at shallow levels within the crust. Geochemistry favours mantle origin of the magma and enrichment of Ba and Sr are indicative of involvement of carbonatite melt in the source region. The study envisages the presence of a juvenile CO 2 enriched upper mantle below the southern Indian continental crust during the Pan-African time.

Counterclockwise exhumation of a hot orogen: The Paleoproterozoic ultrahigh-temperature granulites in the North China Craton

Regional metamorphic belts provide important constraints on the plate tectonic architecture of orogens. We report here a detailed petrologic examination of the sapphirine-bearing ultra-high temperature (UHT) granulites from the Jining Complex within the Khondalite Belt of the North China Craton (NCC). These granulites carry diagnostic UHT assemblages and their microstructures provide robust evidence to trace the prograde, peak and retrograde metamorphic evolution. The P–T conditions of the granulites estimated from X Mg Grt(Mg/Fe + Mg) − X Mg Spr isopleth calculations indicate temperature above 970 °C and pressures close to 7 kbar. We present phase diagrams based on thermodynamic computations to evaluate the mineral assemblages and microstructures and trace the metamorphic trajectory of the rocks. The evolution from Spl–Qtz–Ilm–Crd–Grt–Sil to Spr–Qtz–Crd–Opx–Ilm marks the prograde stage. The Spl–Qtz assemblage appears on the low-pressure side of the P–T space with Spr–Qtz stable at the high-pressure side, possibly representing an increase in pressure corresponding to compression. The spectacular development of sapphirine rims around spinel enclosed in quartz supports this inference. An evaluation of the key UHT assemblages based on model proportion calculation suggests a counterclockwise P–T path. With few exceptions, granulite-facies rocks developed along collisional metamorphic zones have generally been characterized by clockwise exhumation trajectories. Recent evaluation of the P–T paths of metamorphic rocks developed within collisional orogens indicates that in many cases the exhumation trajectories follow the model subduction geotherm, in accordance with a tectonic model in which the metamorphic rocks are subducted and exhumed along a plate boundary. The timing of UHT metamorphism in the NCC ( c. 1.92 Ga) coincides with the assembly of the NCC within the Paleoproterozoic Columbia supercontinent, a process that would have involved subduction of passive margins sediments and closure of the intervening ocean. Thus, the counterclockwise P–T path obtained in this study correlates well with a tectonic model involving subduction and final collisional suturing, with the UHT granulites representing the core of the hot or ultra-hot orogen developed during Columbia amalgamation.

The Precambrian Khondalite Belt in the Daqingshan area, North China Craton: evidence for multiple metamorphic events in the Palaeoproterozoic era

Abstract High-grade pelitic metasedimentary rocks (khondalites) are widely distributed in the northwestern part of the North China Craton and were named the ‘Khondalite Belt’. Prior to the application of zircon geochronology, a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology. We report here SHRIMP U–Pb zircon ages and Hf-isotope data on metamorphosed sedimentary and magmatic rocks at Daqingshan, a typical area of the Khondalite Belt. The main conclusions are as follows: (1) The early Precambrian supracrustal rocks belong to three sequences: a 2.56–2.51 Ga supracrustal unit (the previous Sanggan ‘group’), a 2.51–2.45 Ga supracrustal unit (a portion of the previous upper Wulashan ‘group’) and a 2.0–1.95 Ga supracrustal unit (including the previous lower Wulashan ‘group’, a portion of original upper Wulashan ‘group’ and the original Meidaizhao ‘group’) the units thus do not represent a true stratigraphy; (2) Strong tectono-thermal events occurred during the late Neoarchaean to late Palaeoproterozoic, with four episodes recognized: 2.6–2.5, 2.45–2.37, 2.3–2.0 and 1.95–1.85 Ga, with the latest event being consistent with the assembly of the Palaeoproterozoic supercontinent Columbia; (3) During the late Neoarchaean to late Palaeoproterozoic (2.55–2.5, 2.37 and 2.06 Ga) juvenile, mantle-derived material was added to the crust.

Gravity signatures, derived crustal structure and tectonics of Achankovil Shear Zone, southern India

The kinematics of the crustal-scale Achankovil Shear Zone in the Southern Granulite Terrane, India is often debated. This shear zone is considered to mark the terrane boundary between Madurai Granulite Block towards the north and Kerala Khondalite Belt towards the south. Closely spaced 4500 new gravity measurements in the region reveal the detail crustal fabric across the Achankovil Shear Zone. A subdued Bouguer gravity anomaly along a N–S profile that trends approximately orthogonal to the Achankovil Shear Zone signifies that this part of the crust is devoid of any crustal-scale density discontinuity. Furthermore, horizontal-gradient, analytical signal and second vertical derivative analyses of the gravity data suggest that the density inhomogeneity across the Achankovil Shear Zone is relatively shallow in origin thereby refuting it as a terrane boundary. A prolonged zone of positive gravity gradient, as much as 125 km wide towards south, is due to the effect of the continental margin. 2 1/2D gravity modelling along the profile, constrained from seismic results, reveals a three layer crustal configuration with the depth to Moho varying from 41 km beneath the Vattalkundu to about 34 km beneath the Kanyakumari and then attaining to about 32 km beneath the continental shelf region. The 22 km thick quasi-continental crust in the adjoining Indian Ocean indicates a transitional crust in the Gulf of Mannar region. Analysis of new gravity data thus supports the idea that the Achankovil Shear Zone is an intracratonic litho-tectonic feature and the two provinces across it are related by a continual progression in single metamorphic terrain rather than an ancient geo-suture.

Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

Precise chemical composition, including Fe 3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T &gt; 800-850 °C and P = 5 ± 1 kbar. The full analyses were normalized on the basis of [O 12-(x+y+z) (OH) x Cl y F z ]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H 2 O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe 3+ /Fe tot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. In the host granulite, fine-grained biotite is Fe 3+ -free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and X Mg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X Mg , and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2OH - and the “fluorination” Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

Carbonic metamorphism at ultrahigh-temperatures: Evidence from North China Craton

The North China Craton (NCC) constitutes one of the fundamental Precambrian nuclei of Asia. The Paleoproterozoic khondalite belt along the northern margin of the NCC shows evidence for extreme crustal metamorphism at temperatures exceeding 1000 °C and pressures of 10–12 kbar with the formation of diagnostic ultrahigh-temperature (UHT) mineral assemblages including sapphirine + quartz, high alumina orthopyroxene + sillimanite and high-temperature mesoperthites. Here we record for the first time the common occurrence of CO 2-rich fluid inclusions in peak UHT minerals in the Mg–Al granulites of NCC, and trace the fluid evolution history. From microthermometric and laser Raman spectroscopic studies, we show that the syn-metamorphic fluid in the UHT rocks was pure CO 2. While very high density carbonic fluids are preserved in some cases, corresponding to entrapment at peak conditions, many of the inclusions underwent density modification during the rapid and near-isothermal exhumation of the rocks following peak UHT metamorphism and initial cooling at depth. The present study area within the northern margin of the NCC defines a collisional orogenic belt where continental collision and deep subduction were possibly followed by extensional collapse of the orogen. Evidence for the extensional phase is provided by the several mafic magmatic intrusions of Paleoproterozoic age that are seen emplaced within the granulite facies supracrustals. The thermal anomaly displayed by the UHT granulites may thus correlate with asthenospheric upwelling and emplacement of mafic magmas, possibly associated with plume activity, which provided the heat and CO 2 input to achieve extreme crustal metamorphism and generation of dry granulite facies assemblages. Our study is the first report of syn-metamorphic CO 2-rich fluids in UHT granulites from a Paleoproterozoic (ca. 1.92 Ga) terrane and provide evidence for carbonic metamorphism at ultrahigh-temperatures as North China Craton joined the Columbia supercontinent amalgam.

Palaeoproterozoic Khondalite Belt in the western North China Craton: New evidence from SHRIMP dating and Hf isotope composition of zircons from metamorphic rocks in the Bayan Ul-Helan Mountains area

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan Ul-Helan Mountains area, the western block of the North China Craton (NCC). Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages (T DM1) of 2455 to 2655 Ma (19 analyses). Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 207Pb/206Pb age of 1978±17 Ma. A few detrital zircons are older (2871 to 2469 Ma). The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with T DM1 ages of 1999 to 3047 Ma. In combination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite series in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; (2) The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite (metasedimentary) Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; (3) The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; (4) Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.

Application of CCSEM to heavy mineral deposits: Source of high-Ti ilmenite sand deposits of South Kerala beaches, SW India

For the characterization of sediments, Computer Controlled Scanning Electron Microscopy (CCSEM) is a powerful method in obtaining chemical data on individual mineral grains and modal analysis of the heavy mineral fraction of sediment samples. Here we show how the CCSEM method can be used to evaluate ilmenite ore grade as well as a tool to investigate the source of heavy mineral deposits. The heavy mineral rich deposits in beach sands around the town of Chavara in SW India are characterized by ilmenite with elevated TiO 2 contents, often exceeding 60 wt.%. In order to determine the origin of these high-TiO 2 ilmenite deposits, we collected a series of beach sediment samples (22) from a c. 800 km long stretch of coastline from northern Kerala state to well within the Tamil Nadu state. A set (7) of river sediments was also taken, roughly covering the catchment area to the beach samples. The data show that the sediments in the Chavara high-Ti ilmenite deposit are distinguished by minor elements in ilmenite, garnet chemistry and heavy mineral assemblage: Chavara ilmenite has high MgO and low MnO contents; garnets have low grossular components and the heavy mineral assemblage is dominated by sillimanite–kyanite in addition to ilmenite. These features correlate with basement geology in the hinterland, and with sediments from rivers, draining the basement. Based on these observations we conclude that high-Ti ilmenite from Chavara beaches originates in the khondalite belt of high-grade metasediments. Our study demonstrates rapid mineral analyses in sediments by CCSEM to be efficient in the characterization of mineral compositions and assemblages in sediments, in the identification of possible source regions and thus ultimately in exploration for industrial mineral resources.

Timing of Paleoproterozoic ultrahigh-temperature metamorphism in the North China Craton: Evidence from SHRIMP U–Pb zircon geochronology

We report SHRIMP U–Pb zircon ages from two samples of Mg–Al granulite and one of normal khondalite from the Khondalite belt of Inner Mongolia, North China Craton. These data constrain the timing of extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in the region. The UHT indicator assemblages in these rocks include equilibrium sapphirine + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature perthites. The surrounding khondalites also carry spinel + quartz equilibrium assemblage. Based on the stability of sapphirine + quartz and orthopyroxene + sillimanite + quartz in the matrix assemblages, T > 1000 °C and P > 10 kbar for peak metamorphism was estimated in a previous study, with initial isobaric cooling and subsequent isothermal decompression along a broad anticlockwise trajectory. Metamorphic zircons from the UHT rocks are characterized by ovoid shapes with numerous high-order crystal faces and low luminescence in cathodoluminescence (CL) images. As metamorphic temperatures increase, based on the presence of UHT indicator minerals, relict oscillatory zones are progressively destroyed and the grains become uniformly dark in CL. The oldest detrital cores reveal 207Pb/ 206Pb ages up to 2090 ± 22 Ma, but most have ages of ∼1970 Ma. Importantly, the highest grade sample with sapphirine-bearing UHT assemblage, contains only a single zircon population with a weighted mean 207Pb/ 206Pb age of 1919 ± 10 Ma. This is the same age, within error, as the youngest populations in the other two samples and is interpreted to record the time of UHT metamorphism in the North China Craton. The zircons in the highest-grade sample have unusually high Th/U ratios (average 1.44) for metamorphic zircon, which may prove to be a feature of UHT rocks. The Khondalite belt defines a collisional orogen between the Yinshan block to the north and the Ordos block to the south, that together form the Western Block of the North China Craton. The ∼1.92 Ga UHT metamorphic event in the North China Craton preceded the collision that amalgamated the North China Craton along the Trans-North China Orogen at ∼1.85 Ga, but both coincided with the assembly of the Paleoproterozoic supercontinent Columbia.

When Did Plate Tectonics Begin on the North China Craton? Insights from Metamorphism

Understanding when and why plate tectonics began on Earth is one of the most important unresolved problems in earth sciences. As a final result of plate tectonics, continental collisional orogens are formed, and the identification of such orogens is regarded as a milestone in recognizing the operation of plate tectonics in Earth's history. Metamorphism of continental collisional belts is generally characterized by clockwise p-T paths, especially for those involving isothermal decompression following the peak metamorphism. Therefore, clockwise p-T paths involving isothermal decompression are considered one of the important indicators for recognition of plate tectonics, especially in the early history of the earth. As one of the oldest continental blocks in the world, the North China Craton is considered a promising area for applying the large-scale synthesis approach of metamorphic p-T paths in understanding tectonic settings or processes since extensive investigations have been undertaken on the p-T-t evolution of the basement rocks over the craton. Tectonically, the North China Craton is composed of three small continental blocks (Eastern Block, Yinshan Block, and Ordos Block) and three Paleoproterozoic mobile belts (Trans-North China Orogen, Khondalite Belt, and Jiao-Liao-Ji Belt). Metamorphism of late Archean basement rocks in the Eastern and Yinshan Blocks is characterized by anticlockwise p-T paths, mostly involving isobaric cooling, suggesting that the heating source of the metamorphism was related to the underplating or intrusion of large volumes of mantle-derived magmas. Although theoretically such mantle-derived magmas form under continental magmatic arc, mantle plume or continental rift environments, only a mantle plume model can reasonably explain spatial and temporal distribution, lithological association, and structural features of the late Archean basement rocks in the Eastern and Yinshan Blocks. Therefore, mantle plumes may be the major tectonic mechanism that governed the formation and evolution of the late Archean crust in the Eastern and Yinshan Blocks, and plate tectonics may not have operated in the two blocks during late Archean period. Metamorphism of the basement rocks in the Paleoproterozoic Khondalite Belt and the Trans-North China Orogen is both characterized by clockwise p-T paths involving isothermal decompression following peak metamorphism, which suggests that the two orogens underwent initial crustal thickening followed by rapid exhumation/uplift tectonic processes. Such tectonic processes are typical indicators of collisional orogens that accommodate plate tectonics. The Paleoproterozoic Jiao-Liao-Ji Belt can be divided into the southern and northern zones, of which the former is characterized by anticlockwise p-T paths, whereas the latter is characterized by clockwise p-T paths, also indicating a regime of plate tectonics. The time of the initiation of plate tectonics in the North China Craton can be approximated by the age of the earliest juvenile crustal components in the three Paleoproterozoic tectonic belts. So far, the convincing oldest, subduction-related, juvenile crustal component in the North China Craton is the 2.56 Ga Wutai granitoids in the Trans-North China Orogen, which can be used to approximate the timing of the onset of plate tectonics in the North China Craton.

Discovery of sapphirine-bearing Mg–Al granulites in the North China Craton: Implications for Paleoproterozoic ultrahigh temperature metamorphism

We report here for the first time the occurrence of Mg–Al granulites within the khondalite belt of the North China Craton and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT indicator assemblages in these rocks include equilibrium sapphirine + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature perthites. The surrounding khondalites also carry spinel + quartz equilibrium assemblage. The sapphirines occurring in various microstructural associations are markedly magnesian ( X Mg = 0.69–0.77) with a composition varying from 7:9:3 to 2:2:1 end members. Spinel is a solid solution of Mg-spinel and hercynite ( X Mg = 0.41–0.54) with ZnO content varying from 0.48 to 1.79 wt.%. Garnet shows a compositional range between pyrope and almandine ( X Mg = 0.39–0.48) with minor grossular (2.6–3.8 mol%) and spessartine (0.5–1.8 mol%). The orthopyroxenes display marked compositional variations in terms of Al 2O 3 content, with the core of the medium-grained orthopyroxene adjacent to sillimanite in the matrix showing the highest Al 2O 3 content (8.4 wt.%). The history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine, spinel, quartz and sillimanite within garnet. Sapphirine formed during the peak stage through the reaction: Spl + Qtz + Sil → Spr. Occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction: Spr + Qtz + Grt → Opx + Sil. A series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse-grained minerals such as: Opx + Sil + Qtz → Crd, Grt + Sil + Qtz → Crd, Grt + Qtz → Crd + Opx, Grt + O 2 → Crd + Qtz + Fe 2O 3 in Mag or Ilm, and finally Opx + Kfs + H 2O → Bt + Qtz. Whereas the stability of sapphirine + quartz and orthopyroxene + sillimanite + quartz in the matrix assemblage suggests T > 1000 °C and P > 10 kbar peak metamorphism, conventional geothermobarometric estimates confirm the ultrahigh-temperature nature and also trace the retrograde conditions (930–970 °C at P > 10 kbar — Al in Opx; 900–1000 °C — perthite; 930–990 °C — Grt–Opx; and 910–940 °C — sapphirine–spinel). Electron microprobe dating of monazites in textural association with the UHT assemblages yield a sharply defined isochron age of 1917 ± 48 Ma, which correlates with the 1927 ± 11 Ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel + quartz assemblage. An evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and we interpret these trajectories to be part of an overall anti-clockwise P– T evolution. The study area, located within the northern margin of the North China Craton, defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen. Evidence for the extensional phase is provided by the several mafic magmatic intrusives of Paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle-derived magmas. The UHT metamorphism in North China Craton coincided with the assembly of the Paleoproterozoic supercontinent Columbia.

Effects of temperature, pH and NaCl on protease activity in digestive tract of young turbot,Scophthalmus maximus

Isochemical conversion of garnet-biotite bearing paragneiss to charnockite in the Precambrian Khondalite belt of southern Kerala is described from Ponmudi area. Petrographic evidences indicate the formation of hypersthene by the breakdown of biotite in the presence of quartz following the reaction: Biotite + quartz → hypersthene + K-feldspar + vapour. The estimated pressure — temperature conditions of metamorphism are around 5–7 kbars and 750° ± 40°C. Presence of CO2-rich, mixed CO2-H2O and H2O-rich inclusions were noticed in gneiss as well as in charnockites. Charnockites contain abundant CO2-rich inclusions.

SHRIMP U–Pb zircon geochronology of Palaeoproterozoic metasedimentary rocks in the North China Craton: Evidence for a major Late Palaeoproterozoic tectonothermal event

Although metasedimentary successions are widely distributed across the North China Craton, those of Palaeoproterozoic age are restricted to three orogenic belts, namely: the Trans-North China Orogen, the Khondalite Belt and the Jiao-Liao-Ji Belt. Prior to this work, some of those successions were considered to be Archaean in age. The Palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3.0 to 2.1Ga, with the majority recording an age between 2.3 and 2.0Ga. This latter group define a series of magmatic events that occurred between formation of the Archaean basement and deposition of the metasedimentary rock protoliths. The metasedimentary rocks occur as two associations; older volcano-sedimentary units that formed between 2.37 and 2.0Ma, and younger sedimentary units that formed mainly between 2.0 and 1.88Ga. All these rock associations were then subjected to metamorphism during the Late Palaeoproterozoic (1.88 and 1.85Ga) tectonothermal event, named the Lüliang Movement in early Chinese literature, which was a continent–continent collision that led to the amalgamation of the North China Craton.

Incipient charnockite formation in Kerala (S. India)—Constraints from geochronology and REE in zircon

Incipient charnockite in S. India have been intensively studied over the last three decades, resulting in a number of petrogenetic models based on the whole-rock and mineral chemical and isotopic data as well as fluid inclusion studies. The aim of our study is to date dehydration process and to detect phase transition effects on U–Th–Pb and REE systematic in zircon by comparing zircons separated from closely paired samples on either side of phase transition. We sampled incipient charnockites from number of localities from Kerala Khondalite Belt (KKB). Leptynitic gneisses and khondalite, two major lithologies of the KKB, are common hosts for the incipient charnockite. Three types of incipient charnockite were observed: (1) charnockite patches related to the leucosome formation in leptinitic gneisses, (2) elongated charnockitic patches cross cutting gneissic fabric in khondalite and (3) charnockite as margins to a pegmatitic dyke in porphyritic granite. Early results of examination of the zircons from incipient charnockite and hosting leptynitic gneiss in the Manali quarry (type 1) showed that oscillatory zoning in zircons are generally blurred and broad indicating recrystallisation of protolith zircon. Recrystallisation caused partial resetting of U–Th–Pb isotopic system resulting in large discordance of the obtained ages. Cl-dark metamorphic zircon rims majority of the grains. Zircons in charnockite have nearly identical internal structure however, CL-dark rims are much thicker with weak sector zoning. In addition some grains have two generations of identical rims. Two groups of CL-dark rims in leptynitic gneiss gave concordant ages of 520 ± 10 Ma and 582 ± 22 Ma. Zircon rims from charnockite split in two age groups as well: 503 ± 13 Ma and 555 ± 11 Ma. Younger and older generations of zircon rims from each lithology are coeval within age errors. Older generation of the zircon rims shows a HREE depleted pattern indicating closed system fractionation of the REE between zircon and garnet. Younger generation of zircon rims are most probably related to partial melting and leucosome formation therefore they date charnockite formation. However, depleted HREE pattern in these zircons are not consistent with open system partitioning.

Pan-African granitic magmatism in the Kerala Khondalite Belt, southern India

Granitic rocks in the Kerala Khondalite Belt (KKB) of southern India can be distinguished according to their chemical composition and their mode of formation. Porphyritic granites, now present as coarse-grained augen gneisses, represent an early stage of Pan-African granitic magmatism, which most likely occurred at amphibolite-facies conditions. U–Pb and EPMA monazite dating suggest a minimum intrusion age of 590 Ma for the granitic precursors of the augen gneisses. Garnet- and biotite-bearing leucogranites are abundant throughout the whole KKB and were formed from high-T dehydration–melting of the various KKB gneisses at granulite-facies conditions subsequent to the main stage of Pan-African ductile deformation. Fine-grained granites, some of them with distinct alkali affinities, are abundant in the Achankovil Unit and formed pre- to post-tectonically. Available geochronological data point to an emplacement c. 560 Ma ago or later. Corresponding petrological and field data suggest that the leucogranites were formed and emplaced during the peak stage of Pan-African metamorphism, whereas the Achankovil granites partly may reflect extensional tectonics and lithospheric thinning.

Composite nature of the North China Granulite-Facies Belt: Tectonothermal and geochronological constraints

Granulite-facies rocks are intermittently exposed in a roughly E–W trending belt that extends for approximately 2000 km across the North China Craton, from the Helanshan, Qianlishan, Wulashan–Daqingshan, Guyang and Jining Complexes in the Western Block, through the Huai'an, Hengshan, Xuanhua and Chengde Complexes in the Trans-North China Orogen, to the Jianping (Western Liaoning), Eastern Hebei, Northern Liaoning and Southern Jilin Complexes in the Eastern Block. The belt is generally referred to as the North China Granulite-Facies Belt, previously interpreted as the lowest part of an obliquely exposed crust of the North China Craton. Recent data indicate that the North China Granulite-Facies Belt is not a single terrane. Instead, it represents components of three separate terranes: the Eastern and Western Blocks and Trans-North China Orogen. Each of these units records different metamorphic histories and reflect the complex tectonic evolution of the NCC during the late Archean and Paleoproterozoic. Mafic granulites in the Eastern Block and the Yinshan Terrane (Western Block) underwent medium-pressure granulite-facies metamorphism at about ∼2.5 Ga, with anticlockwise P–T paths involving near isobaric cooling following peak metamorphism, reflecting an origin related to intrusion and underplating of mantle-derived magmas. Pelitic granulites in the Khondalite Belt (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.0–1.9 Ga, with clockwise P–T paths, which record the Paleoproterozoic amalgamation of the Yinshan and Ordos Terranes to form the Western Block. Mafic and pelitic granulites in the Trans-North China Orogen experienced high- to medium-pressure granulite-facies metamorphism at ∼1.85 Ga, with clockwise P–T paths involving nearly isothermal decompression following peak metamorphism, which are in accord with the final collision between the Eastern and Western Blocks to form the North China Craton at ∼1.8 Ga. The NCGB cannot therefore represent a separate unique terrane; instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times.

Geochronology of the khondalite belt of Trivandrum Block, Southern India: Electron probe ages and implications for Gondwana tectonics

In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma. The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities. The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.

Constraints on the Timing of Pan‐African Granulite‐Facies Metamorphism in the Kerala Khondalite Belt of Southern India: SHRIMP Mineral Ages and Nd Isotopic Systematics

The supracrustal Kerala Khondalite Belt (KKB; southern India) constitutes an important but little understood terrain in the evolution of East Gondwana. Here we report SHRIMP U‐Pb zircon and titanite ages for a suite of metasedimentary rocks from the KKB. Metamorphic ages at $$530.2\pm 4.4$$ and $$525\pm 4.5$$ Ma were obtained from metapelitic and metapsammitic migmatites, respectively. These ages are interpreted to date peak metamorphism because both rock types have extensively melted under granulite‐facies conditions. A similar age ($$520\pm 4.7$$ Ma) was obtained from titanite from a granulite‐facies calc‐silicate rock. These ages are within error of SHRIMP ages for granulite‐facies assemblages in Lützow‐Holm Bay, East Antarctica, the Highland and Wanni Complexes of Sri Lanka, and Southern Madagascar. We propose that this timing reflects a short span (∼10 m.yr.) of high‐temperature granulite‐facies metamorphism and concomitant widespread migmatization that marks the final stage of collision between East and West Gondwana.

Late Archean to Paleoproterozoic evolution of the North China Craton: key issues revisited

A recently proposed model for the evolution of the North China Craton envisages discrete Eastern and Western Blocks that developed independently during the Archean and collided along the Trans-North China Orogen during a Paleoproterozoic orogenic event. This model has been further refined and modified by new structural, petrological and geochronological data obtained over the past few years. These new data indicate that the Western Block formed by amalgamation of the Ordos Block in the south and the Yinshan Block in the north along the east-west-trending Khondalite Belt some time before the collision of the Western and Eastern Blocks. The data also suggest that the Eastern Block underwent Paleoproterozoic rifting along its eastern continental margin in the period 2.2–1.9 Ga, and was accompanied by deposition of the Fenzishan and Jingshan Groups in Eastern Shandong, South and North Liaohe Groups in Liaoning, Laoling and Ji’an Groups in Southern Jilin, and possibly the Macheonayeong Group in North Korea. The final closure of this rift system at ∼1.9 Ga led to the formation of the Jiao-Liao-Ji Belt. In the late Archean to early Paleoproterozoic, the western margin of the Eastern Block faced a major ocean, and the east-dipping subduction beneath the western margin of the Eastern Block led to the formation of magmatic arcs that were subsequently incorporated into the Trans-North China Orogen. Continued subduction resulted in a major continent-continent collision, leading to extensive thrusting and high-pressure metamorphism. The available age data for metamorphism and deformation in the Trans-North China Orogen indicate that this collisional event occurred at about 1.85 Ga ago, resulting in the formation of the Trans-North China Orogen and final amalgamation of the North China Craton.